hCAPTCHA looks interesting, although it seems they use Blockchain for no real reason compared to just storing the payments as rows (i.e what they gain from being chained on top of another)
This is also why Git’s history is easy to edit when it’s only on your machine. But once you push to GitHub and others clone your repo, it becomes a lot harder to edit history. Yes, Git isn’t a blockchain, but it does use the idea of hashing the previous “block” (commit) and storing it in the current “block.”